Rubber products are often prepared by assembling a multiplicity of uncured rubber components, although sometimes a combination of uncured and cured rubber components, which desirably requires them to have a somewhat tacky surface to enhance the stability of their assembly, followed by curing the resulting assembly under conditions of elevated temperature and pressure. Such tack is often referred to as building tack. The use of building tack in fabricating rubber products is well known to those skilled in such art. Usually building tack is used in the context of uncured rubber and, in this description, it is also used in the context of partially cured rubber. In this description, cured rubber is normally equated with partially cured rubber for the treatment purposes.
For such purpose, a conventional practice is to treat at least one of the uncured rubber surfaces with a solvent in order to both make the rubber surface tacky and, of a usually equal importance, to remove surface bloom from the rubber surface and/or freshen the rubber surface. Sometimes, the surface is treated with a rubber cement based on such solvent. The surface bloom is often in the form of various oils, fatty acid salts, antidegradants and waxes which have migrated to the surface of the rubber over a period of time and which often tend to inhibit a strong cure between the assembled rubber component surfaces.
In the practice of this invention, removal, or substantial removal, of the surface bloom is a primary consideration.
Of sometimes equal importance, freshening the rubber surface is a primary consideration. In some circumstances, where the uncured rubber surface has been exposed to the atmosphere, such as, for example, storage while the uncured rubber awaits final processing or building into a rubber product, the surface can become contaminated with foreign materials, such as dust or chemicals or by aging or oxidation. Freshening the rubber surface, as referenced herein, is intended to mean the substantial or complete removal of foreign materials and/or oxidation from the rubber surface by solvent treatment and thereby "freshening" the surface. While such freshening does not necessarily increase tack, it is intended that it enhance cured adhesion (to reduce interfacial adhesion failure) as compared to a contaminated uncured rubber surface which has not been freshened.
While various aromatic solvents have heretofore demonstrated an ability to provide adequate building tack for uncured rubber surfaces, it is desired to provide an essentially aliphatic solvent for such surface treatment which has an adequate drying time, essentially eliminates or removes uncured rubber surface bloom, provides uncured rubber surface building tack, provides or enables an interfacial rubber surface adhesion after curing the rubber assembly and contains less than about one weight percent aromatic components.
Accordingly, it is desired to provide a solvent for such purposes which contains less than one weight percent and, thus, is basically free, of aromatic materials such as, for example, benzene, alkyl substituted benzenes, xylene and alkyl substituted xylenes as designated aromatic and substituted aromatic hydrocarbons. It is also desired that it contains less than about one weight percent n-hexane or cyclohexane.
It is desirably required of such solvent that it effectively tackifies rubber surfaces, particularly uncured rubber surfaces, to provide adequate building tack so as to hold the rubber components together prior to curing the assembly and, further, to have a satisfactory short drying time in a practical application.
The solvent ability of a solvent is considered to be important for removing surface bloom from the rubber surface. It is believed that a suitable test for such solvent ability is a Kauri-Butanol (KG) test.
By experience, it is considered herein that a hydrocarbon solvent or solvent mixture having a KG value in a range of about 25 to about 38, preferably about 25 to about 34, is generally satisfactory for the solvency purposes of this invention.
The KG (Kauri-Butanol) values can be conventionally determined by first dissolving 100 grams of Kauri Gum in 500 grams of butanol (thus, the designation KG). The desired solvent, or solvent mixture, is titrated into the KG solution to determine its KG value. KG values have been published in the literature for various aromatic and aliphatic hydrocarbon solvents. The KG value is, generally, an indication of the solubility of the Kauri Gum resin (contained in the butanol solution) in the solvent being tested.
For example, a KG value for toluene or benzene would be about 105-110; cyclohexane about 100; and n-heptane would be about 29.
A reference to the KG test may be found in "Physical And Chemical Examination of Paints, Varnishes, Lacquers And Colors", Eleventh Edition, 1950, pages 449-451, by Gardner and Sward, originally distributed by The Henry Gardner Laboratory, Inc., Bethesda, Md.
Various solvent systems were evaluated for use in treating uncured rubber to aid in the fabrication of rubber products.
The n-heptane, an aliphatic C.sub.7 hydrocarbon with a KG value of about 29, was previously evaluated. However, the n-heptane was considered to be inappropriate because its observed drying time was found to be unacceptable because it was too slow and it was observed to not provide adequate building tack for the treated uncured rubber surface.
The n-heptane was, however, considered to be a good solvent if it could possibly be successfully used as a blend with other hydrocarbons because of its nearly adequate drying time and its KG value.
It was decided to proceed to evaluate hydrocarbon solvent blends which had a n-heptane base for the rubber tackifying purposes, which would have the properties of an adequate drying time, bloom removal and the providing of an observed satisfactory building tack for a rubber surface, particularly an uncured rubber surface.
It was previously found by the inventors that a solvent blend having a KG value in a range of about 32 to about 38 and comprised of (A) about 15 to about 25 parts by weight n-heptane; and (B) about 55 to about 65 parts by weight of at least one of methylcyclohexane and cis 1,2-methylcyclopentane could be used to treat uncured rubber surfaces to remove surface bloom and enhance or maintain tack. However, such a solvent blend was subsequently observed to have a somewhat objectional odor in the workplace. Thus, it remains desirable to discover additional solvent blends which can satisfactorily achieve the surface bloom and enhance or maintain the uncured rubber surface tack.
In the description of this invention, for the treatment purposes, uncured rubber means uncured compounded rubber. Compounded rubber is used in a conventional sense, namely, for rubber which has been mixed with compounding ingredients.